I mean, sure. I've done keto, I've fasted for several days, it's fine. I personally would have eaten some cruciferous vegetables since I don't trust multivitamins to provide everything, but sounds like he did fine.

My only curiosity as a strategy for months or over a year is if the lack of any protein whatsoever has harmful effects.

I was under the impression that some level of protein intake was necessary to maintain muscles -- something that burning fat doesn't provide.

I'm wondering if he had any muscle loss problems? Or if his leg and back muscles were already so extremely overdeveloped to support his original weight (as is normal), that as they naturally decreased, their protein was made available for normal baseline muscular maintenance?